This invention relates to fibrous bodies, to the manufacture of fibrous bodies, and to methods of applying binders to fibrous products; and more particularly to methods of distributing binders in such products. More specifically, the invention relates to a process for bonding multi-filament strands in a body of desired form, in which the filaments are of fibrous glass and are gathered into strands immediately subsequent to the filament forming operation, the strands then being collected and arranged in a body.
Mats of fibrous glass, because of the inherent glass properties, especially those of strength and inertness, have many uses. They have been employed as filtering, acoustical and thermal insulating media. They also serve effectively for roofing sheets, nonwoven fabrics, and for reinforcing plastic products such as electrical grade laminates.
In some instances, the mats are composed of short fibers held together by a binder. In others the mats are bonded webs of chopped fibrous glass strands. Bundles or strands of continuous glass filaments have also been disposed in mat form. Strands of filaments have superior strength because of the continuous nature of the filaments and their concentrated linear association in strant form. Accordingly, fibrous glass strands are a most desirable constituent where strength is a prime consideration.
Various resinous materials including dispersions, emulsions, solutions, and dry granular powders have been applied to fibrous glass masses as bonding materials to give the finished products dimensional stability and strength. Such liquids or powdered resins have been added in various ways such as by wiping, spraying, submersion and dusting. Obtaining a desired distribution of binding material in the mat at a reasonable cost has posed a problem which has been approached in the prior art in a variety of different ways.
A commonly employed method has been to spray a water dispersion or suspension of the resinous material upon the fibrous glass immediately adjacent the fiber forming operation. This method provides a mat or other body, in which the fibers are massed, having fairly uniform binder distribution and good strength. However, an excessive amount of water is involved and it is necessary to drive off through heating a major portion of this water. The heating requires considerable time and is otherwise costly from a production standpoint.
The same problem of a high water content is faced when a liquid binder is applied by spraying or by dipping to a mat of chopped strands or continuous strands.
Another former method has been to submit the mat of fibers to a scattering of a powdered resinous material. This granular binder has been necessarily shaken down into the mat by vibration and later set by application of heat. In some instances it is necessary to tear the mat apart to mix the resin particles. The mats so treated tend to have poorly bonded areas reducing tensile strength, and high ignition losses, indicating nonuniform distribution and an excessive binder component.
There has also been introduced to the art in U.S. Pat. No. 3,318,746, issued May 9, 1967, and in Canadian Pat. No. 756,563, issued Apr. 11, 1967, two methods for overcoming the high water content problem and obtaining substantially uniform distribution of the binder throughout the mat-like mass.
In U.S. Pat. No. 3,318,746, a liquid binder is first applied longitudinally to the individual filaments before they are gathered into strand form. The strands are then collected in a body of desired configuration such as a mat and the liquid binder in the body is dried in a graduated manner whereby the binder adjacent one surface is comparatively dry and the opposite surface is comparatively wet. A powdered binder is directed against the comparatively dry surface and toward the comparatively wet surface. The powdered binder is distributed and retained substantially uniformly through the body due to the increasing proportion of the binder caught by the progressively wetter portions while the amount of binder decreases through the progressive retention of the binder particles as they pass through the drier portions of the body. Thus there is a substantially uniform distribution of binder in the mat without having an excessive amount of moisture retained in the mat which must be driven off when the binder is being cured.
In the Canadian Pat. No. 756,563, a mat of randomly oriented glass strands is formed on a foraminous conveyor. A water dispersion of solid resin particles is flowed through the mat on the foraminous conveyor to leave resin particles distributed throughout the mat. After the water dispersion of binder particles is passed through the mat, the conveyor passes over a suction box which sucks excess liquid from the conveyor and the mat. The mat, with the small amount of water left therein in the form of droplets, is passed through an oven to reduce the droplets in size and bring the particles of resin in around the area of contact between the fibers to bridge the fibers at the point of contact. Again a substantially uniform distribution of binder has been obtained without an excessive amount of water being left in the mat which must be driven off before the binder can be cured.
The above and other methods not mentioned herein are now being used to achieve substantially uniform distribution of binder throughout the entire fibrous body, but we have discovered that improved characteristics for certain applications can be achieved by selectively distributing binder throughout a mat-like mass of strands. The distribution is uniform in each of at least two different areas, but the binder percentage content or amount of binder solids of each area is different. Mats of fibrous bodies used, for example, as reinforcement in the manufacture of electrical grade laminates, perform better when there is a concentration of or a higher percentage binder content adjacent to at least one surface of a mat or mats used for reinforcement of the laminates.
The dielectric strength, dielectric constant, dielectric breakdown and arc resistance, and dissipation factors of an electrical grade laminate made from such reinforcing mats are substantially improved over such laminates made from mats having a binder solids content which is uniform throughout the thickness thereof and which do not have one or more at least partially filamentized layers. Moreover, the distribution of binder and filamentization of strands as described improves the handleability, unrollability, and the strength of such mats. These improvements have been obtained in the above-described product while still enabling a reduction of fifty percent in the amount of binder solids required to attain the improved properties.
None of the hereinbefore described methods or other known methods for applying binders to fibrous products will permit a high degree of controlled or selective distribution of binder in a mat-like mass to obtain one layer which has a higher percentage binder content than one or more other layers also included in the mat. We have discovered that a preferred method for making a mat having the desired improved characteristics includes the steps of manufacturing a multi-layer mat in which at least one of the layers has strands with a different number of filaments to provide a faster reopening capability than the other layer or layers in the mat. Even with the preferred method of manufacturing the mat, however, none of the known methods of binder distribution would selectively distribute the binder as desired.
Multi-layer strand mats by themselves are not new to the art, having been introduced, for example, in U.S. Pat. No. 3,442,751, issued May 6, 1969. However, there has been no appreciation of the improved characteristics than may be obtained in the product of this invention, nor any attempt to selectively distribute binder throughout the mat to obtain a higher percentage binder content in one or more of the layers of the multi-layer mats than is present in other layers.
It was further discovered that it is possible to filamentize or open the strands in one layer, or less than all the layers, of the multi-layer mat in place, preferably on a collection surface. This novel method of selectively filamentizing strands of a mat and the apparatus for carrying out the method is used to entrap or retain more binder particles in the resulting changed mesh size of a filamentized layer therefrom to provide a higher bond percentage binder content in that layer than in the remaining portion of the mat.
In a co-pending application Ser. No. 201,002, files Nov. 22, 1971, now U.S. Pat. No. 3,760,458 and assigned to the same assignee as the present invention, there is disclosed a method for filamentizing or reopening the strands in the entire body of a mat-like mass or collection of strands to filamentize all of the strands in the body to provide a uniform dispersion of filaments to achieve a more uniform porosity and integrity throughout the entire mat. This process, however, distributes binder uniformly throughout the mat and does not consider a method for selectively distributing the binder in the mat to obtain a higher percentage binder content in individual layers thereof and the product does not provide the unique characteristics of the product disclosed herein.
Accordingly, it is an object of this invention to provide a new method and apparatus for producing a novel fibrous body which advantageously may have binder selectively distributed therein to obtain a higher percentage binder concentration in a predetermined portion of the mat.
It is another object of this invention to provide a new and improved method and apparatus for selectively distributing particles of a solid binder material throughout a mat of glass fibers.
Still another object of the invention is the provision of new and improved method and apparatus for applying binder to mats in such a manner that the handleability of the mats will be improved over that of the prior art processing, in which the performance of an object reinforced by such a mat is improved over prior art products, and to use a smaller amount of binder to accomplish the above objectives than has been used heretofore.
It is still further object of this invention to provide improved fibrous bodies which enhance the physical, mechanical and electrical properties of a composite moled laminate or structure using the fibrous bodies for reinforcement, and to provide an improved laminate product and a novel process for making same.
Another object is the provision of an improved continuous process and apparatus for economically producing bodies of fibrous glass.